1. Field of the Invention
The present invention relates to an FM signal optical transmission apparatus and an FM signal optical reception I! apparatus used for, for example, optical communication, CATV, optical measurement, or mobile communication.
2. Description of the Related Art
In recent years, it has been practically used to perform optical transmission of video or audio data for many channels by making the most use of the small loss and wide-band characteristic of an optical fiber for a video monitoring system, CATV, subscriber system, or mobile communication. The above optical transmission system electrically multiplexes signals of many channels by a plurality of subcarriers having frequencies different from each other and converts them into AM signals and then, directly modulates a semiconductor laser beam or the like by the AM signals to convert the semiconductor laser into an optical signal and transmits the optical signal through an optical fiber. Optical transmission of an AM signal is characterized in that the structure of a MODEM can be simplified particularly for transmission of a video signal and the cost of it can be decreased.
However, the above optical transmission system has the following problems. That is, in the case of video optical transmission, it is necessary to secure a high C/N (carrier-to-noise ratio) in order to secure a desired signal characteristic (such as video quality). Moreover, to obtain a high C/N for video optical transmission of an AM signal, the receiver side indispensably requires a high optical input power.
Moreover, in the case of mobile communication, because the intensity level of an audio or data signal to be transmitted is greatly fluctuated due to movement of a terminal, a high dynamic range is necessary for signal fluctuation. Furthermore, mobile communication is subject to a distortion due to a reflected wave at the time of optical conversion by a semiconductor laser or under transmission through an optical fiber. Furthermore, an AM-signal amplifier requires an amplifier having a high linearity.
To solve the above problems and improve the distortion resistance and noise resistance, an optical transmission system has been proposed so far which simultaneously converts subcarrier-multiplexed AM signals into FM signals and optically transmits them. Moreover, to obtain a desired C/N by increasing the modulation index of the proposed optical transmission system, it is also proposed to obtain an FM signal having a high modulation index by directly modulating the frequency of a semiconductor laser. FIG. 17 shows the structure of an optical transmission system improved as described above.
This optical transmission system outputs an optical-frequency modulated signal by directly modulating a semiconductor laser 41 with a multichannel AM signal (e.g. AM video signals) 30 in an AM-to-FM conversion section 82 of an optical transmitter 81. In this case, by modulating the semiconductor laser 41 into the AM signal 30, not only the amplitude of light is modulated but also the oscillation frequency of the light is modulated. By generating the light having an oscillation frequency slightly different from that of the optical-frequency-modulated signal thus generated by a local-oscillation light source 42 and multiplexing the light and the above optical-frequency-modulated signal by a multiplexer 43 and thereafter, inputting the multiplexed light to a photodiode 44 and heterodyne-detecting the light, a wide-band (e.g. 1 to 6GHz) FM-modulated signal is generated as a beat signal of two lasers and output to an electricity-to-light conversion section 83.
The electricity-to-light conversion section 83 inputs the FM-modulated signal to a semiconductor-laser driving amplifier 88, directly modulates a semiconductor laser 89 for transmission in accordance with the output of the semiconductor-laser driving amplifier 88 to generate an FM optical signal, and transmits the FM optical signal to an optical-fiber cable 92. (The above structure is disclosed in, for example, Japan Patent No. 2700622.)
The FM optical signal transmitted to the optical-fiber cable 92 is amplified by an amplifier (not illustrated) set to the middle of the optical-fiber cable 92 or the like and thereafter, optical-fiber-transmitted to each light-receiving section 93 through an optical turnout (not illustrated) set to the middle of the optical-fiber cable 92.
The light-receiving section 93 first converts an FM optical signal into an electric signal and amplifies the electric signal by a light-to-electricity converter 96 and a preamplifier 97 constituting a light-to-electricity conversion section 95 and thereafter, demodulates the electric signal to an AM signal 31 by an FM-to-AM demodulation section 94. The FM-to-AM demodulation section 94 is a delay-type demodulation circuit which is constituted with high-speed logic ICs 51 and 53 (e.g. AND gate), a delay section 52, and a low-pass filter 54 through a limiter amplifier 50 and realizes wide-band demodulation.
In the case of the above conventional FM transmission system, however, when converting an AM video multiplexed signal into an FM optical signal by the semiconductor laser 41, the CNR (carrier-to-noise ratio) is greatly deteriorated because phase noises of the semiconductor layer 41 are added to the FM optical signal. Therefore, even if the light-receiving intensity of the optical receiver 93 is raised, the sensitivity is only improved up to a certain CNR value. To obtain a desired CNR from the optical receiver 93, it is necessary to use a semiconductor laser having a line width of approx. {fraction (1/10)} the line width of the above conventional system and a semiconductor laser having an external resonator structure. Therefore, there are problems that these semiconductor lasers are expensive and moreover, a plurality of semiconductor lasers must be used.
Moreover, a method is considered which directly converts an AM signal into an electric FM signal in a low frequency band. However, when increasing the modulation index of an FM modulator (modulation factor xe2x89xa710%), problems occur that a distortion occurs in an FM modulator, the signal quality is deteriorated due to the distortion, and thereby, preferable optical transmission cannot be realized.
Moreover, a conventional optical transmission system has problems that, because of transmitting wide-band FM signals of 1 to 6 GHz, the uniformity of frequency bands of signals is broken due to the delay characteristic of parts of an amplifier in the optical transmitter 81 or those of the optical-fiber cable 92 and a distortion due to a phase delay occurs in the AM signal 31 demodulated by the light-receiving section 93.
The present invention is made to solve the above problems and its object is to provide an FM signal optical transmission apparatus and an FM signal optical reception apparatus having a simple structure, a low cost, and a high reception sensitivity compared to conventional ones.
The 1st invention of the present invention is an FM signal optical transmission apparatus comprising
modulation means for converting a multiple signal obtained by subcarrier-multiplexing a plurality of signals into an FM signal having a predetermined carrier frequency;
frequency conversion means for shifting an FM signal converted by said modulation means to a frequency lower than said carrier frequency; and
optical modulation means for converting an optical signal into an FM optical signal by modulating said optical signal in accordance with an FM signal output from said frequency conversion means and transmitting said FM optical signal through an optical fiber cable.
According to the above structure, an optical signal to be transmitted is obtained by performing narrow-band FM modulation and low-frequency conversion, and then intensity modulation. So neither additional circuit nor optical heterodyne detection circuit are necessary. Therefore, the circuit structure is simple compared to a conventional example and superior in stability and reliability. Thus, it is possible to provide an inexpensive FM signal transmission system. Moreover, because of using narrow-band FM signals, modulation distortion hardly occurs in the above modulation means. Therefore, the signal quality is not deteriorated.
The 3rd invention of the present invention is the FM signal optical transmission apparatus according to said 1st invention, further comprising: multiplication means for multiplying a plurality of said FM signals output from said modulation means to output a multiplied signal; and first-band filtering means for band-filtering a desired narrow-band FM signal among multiplied signals output from said multiplication means to output it to said frequency conversion means.
Moreover, according to the above structure, an optical signal to be transmitted is obtained by performing narrow-band FM modulation and low-frequency conversion, and then intensity modulation. Therefore, neither additional circuit nor optical heterodyne detection circuit are necessary. Thus, it is possible to provide an inexpensive FM signal transmission system having a simple circuit structure compared to an conventional example and superior in stability and reliability. Furthermore, because of using narrow-band FM signals, modulation distortion hardly occurs in the above modulation means and thereby, the signal quality is not deteriorated. Furthermore, because of multiplying narrow-band FM signals, it is possible to obtain an FM signal having a large modulation index compared to the case of a conventional example and obtain a desired CNR from an optical receiver.
The 4th invention of the present invention is the FM signal optical transmission apparatus according to said 1st or 3rd invention , further comprising: second-band filtering means for band-filtering only either of a first upper-sideband and a first lower-sideband included in the first sideband of said FM signal output from said modulation means.
Furthermore, according to the above structure, because transmission can be performed at a lower band, it is possible to raise the transmission efficiency and the power consumption for transmission driving is reduced.
The 5th invention of the present invention is the FM signal optical transmission apparatus according to any one of said 1st to 4th inventions, wherein said modulation means is a voltage-control oscillator or relaxation oscillator.
The 6th invention of the present invention is the FM signal optical transmission apparatus according to any one of said 1st to 4th inventions, wherein said modulation means phase-modulates said multiple signal to convert it into a phase-modulated signal and thereafter, multiplexes the phase-modulated signal and said carrier signal and thereby, converts the multiplexed signal into a narrow-band FM signal to output it.
Furthermore, according to the above structure, it is possible to simplify a circuit structure compared to the case of a conventional example and provide an inexpensive system.
The 7th invention of the present invention is the FM signal optical transmission apparatus according to any one of said 1st to 6th inventions, wherein suppression means for suppressing the central frequency of said FM signal output from said modulation means is included, and the output from said suppression means is input to said frequency conversion means to serve as an object of said shifting.
Furthermore, according to the above structure, it is possible to improve the quality of transmission signals compared to the conventional quality though the structure is simple.
The 9th invention of the present invention is the FM signal optical transmission apparatus according to any one of said 1st to 8th inventions, wherein compensation means is included which applies dispersion compensation or group-delay compensation to said FM signal to be transmitted.
Furthermore, according to the above structure, it is possible to inexpensively realize an optical transmission system superior in phase characteristic under transmission.
The 15th invention of the present invention is an FM signal optical reception apparatus comprising compensation means for dispersion-compensating or group-delay-compensating an FM optical signal transmitted through an optical fiber; light-to-electricity conversion means for converting an FM optical signal output from said compensation means into an FM electric signal; and demodulation means for demodulating said FM signal converted by said light-to-electricity conversion means into an AM signal.
Furthermore, according to the above structure, it is possible to inexpensively realize an optical reception system capable of suppressing a distortion due to a phase shift under transmission.